Showing papers by "Walter Collischonn published in 2003"

Long-term flow forecasts based on climate and hydrologic modeling: Uruguay River basin

Abstract: [1] This paper describes a procedure for predicting seasonal flow in the Rio Uruguay drainage basin (area 75,000 km2, lying in Brazilian territory), using sequences of future daily rainfall given by the global climate model (GCM) of the Brazilian agency for climate prediction (Centro de Previsao de Tempo e Clima, or CPTEC). Sequences of future daily rainfall given by this model were used as input to a rainfall-runoff model appropriate for large drainage basins. Forecasts of flow in the Rio Uruguay were made for the period 1995–2001 of the full record, which began in 1940. Analysis showed that GCM forecasts underestimated rainfall over almost all the basin, particularly in winter, although interannual variability in regional rainfall was reproduced relatively well. A statistical procedure was used to correct for the underestimation of rainfall. When the corrected rainfall sequences were transformed to flow by the hydrologic model, forecasts of flow in the Rio Uruguay basin were better than forecasts based on historic mean or median flows by 37% for monthly flows and by 54% for 3-monthly flows.

Variabilidade Temporal no Regime Hidrológico da Bacia do Rio Paraguai

Abstract: Analysis of flow measured at 20 sites, rainfall measured at 36 sites, and a 95-year record of water-levels at a site in the basin of the river Paraguay (area 1095 10 3 km 2) showed that the flow regime during the approximate period 1960-70 differed substantially from the flow regime both before and after. The long record of Ladario water-levels suggested that the changes between one period and the next were abrupt, and that the periods differed not only in terms of mean water-levels but also in terms of the year-to-year correlational structure within the record. Despite the fragmentary nature of rainfall records from 36 sites, an explanation for the increased flows since 1970 was found in the increased rainfall, as assessed in terms of the frequency of annual rainfalls rather than the long-term mean rainfall. There was some degree of consistency in the change of rainfall pattern across the Paraguay basin as a whole. A detailed examination of daily rainfall characteristics at two gauges where records were fairly complete showed that during the 1960-70 period, when river flows were low, dry spells were more persistent and, on days when rain did fall, the amounts of rain were generally smaller. The results obtained were compared with results obtained by other researchers using flow records from the Rivers Paraguay, Paraná, Negro and Uruguay in the la Plata basin, and rainfall records from other parts of South America. There is now strong evidence of changes in the runoff regime of the la Plata basin during the last 40 years, not all of which can be attributed to land-use change, as there is complementary evidence of change in rainfall regime. The results were also compared with findings from the Congo basin, which appears to exhibit changes in flow regime that are a mirror image of those found for the Paraguay at Ladario. Key-words: hydrological change; rio Paraguay.

Comparison of flow forecasts by runoff models: a case study from southern Brazil

Abstract: Over 80% of Brazil's energy demand is supplied at present by hydropower. It is therefore essential for purposes of future planning to have good estimates of reservoir inflows, and the longer the lead time of streamflow forecasts, the more distant is the planning horizon for decisions concerning the costing, production and distribution of power. Whilst it is relatively easy to forecast river basin response to precipitation that has already fallen, forecasts for periods beyond this immediate response time must be based upon estimates of future precipitation. This paper describes the results of work which combines forecasts of future rainfall given by a model of global climate with a detailed, distributed model of river basin response in the Brazilian part of the River Uruguay basin, an area of intensive hydropower production. Comparison of predicted and observed rainfall over the period of model calibration showed that it was necessary to apply a statistical correction to eliminate a spatial trend in predicted rainfall. Data from a two-year period were set aside for model validation. Results show that the combined use of models of climate and river-basin response has been successful for the River Uruguay basin, in that the root mean square error (RMSE), between observed flows and flows predicted by the combined model, is substantially less than the RMSE between observed flows and the historic mean monthly flows.